Fossil Discoveries in Downtown San Diego

A mammoth tusk is discovered during an excavation for a new building

By N. Scott Rugh

The following story is contributed by the San Diego Natural History Museum, one of Natural History magazine’s Museum Partners. Members of any of our partner organizations receive Natural History as a benefit of their museum membership. Founded in 1874, the San Diego Society of Natural History is the oldest scientific institution in southern California, and the third oldest west of the Mississippi. Its mission is to interpret the natural world through research, education and exhibits; to promote understanding of the evolution and diversity of southern California and the peninsula of Baja California; and to inspire in all a respect for nature and the environment. Scientists at the Museum are actively engaged in research programs and regularly publish in peer-reviewed scientific journals. The extensive scientific collections of the Biodiversity Research Center of the Californias (BRCC) support the research of many scientific disciplines, including those working to define and preserve biodiversity and monitor global change.

View of the 16th and Market project site showing the mammoth excavation in the northwest corner.

Kesler Randall and Sarah Siren of the Museum’s PaleoServices Dept.

During the summer of 2007, field staff from the PaleoServices Department of the San Diego Natural History Museum monitored daily the excavation for a new building in downtown San Diego at the corner of 16th and Market Streets for signs of fossils. The owner of the project, Father Joe Carroll of Saint Vincent de Paul Management, Inc., was building a low-income family complex: the Saint Vincent de Paul Village high-rise. Most of July and August passed, and no fossils were seen. A local environmental consulting company, SCS Engineers, was also contracted by the owner of the property, and oversaw the removal of 12,000 tons of contaminated soil (hazardous and non-hazardous waste) from the project site. Toward the end of the excavation, on August 23, 2007, SCS staff member Bob Gutzler stopped by to check on the progress. The excavation had just reached the final grade, when one of the grade checkers asked Bob about a small patch of white material exposed near the northwest corner of the pit. Fortunately, before working at SCS, Gutzler had been a long-time staff member of the Paleontology Department at the San Diego Natural History Museum and recognized the small patch of white material as part of a large tusk. The fossil was later identified as the tusk of a mammoth. For the next several days, about half of the staff of the Paleontology Department worked diligently in the 85-degree heat of late summer to get the eight-foot-long tusk jacketed with plaster and removed from the 30-foot-deep pit.

A nicely preserved pair of the extinct bay scallop, Argopecten abietis, found in the northeast corner of the Thomas Jefferson School of Law excavation.

Kesler Randall and Sarah Siren of the Museum’s PaleoServices Dept.

Species of mammoths, Mammuthus spp., existed from the early Pliocene (4.8 million years before present) to the Holocene (4000 years before present). If mammoths were alive today, they would look familiar to most of us. The bone and tooth structure of mammoth fossils are very similar to those of modern-day elephants. Mammoths and elephants belong to the same family, Elephantidae, of the Order Proboscidea. Like elephants, mammoths had vertically laminated, flat-topped molars adapted for chewing tough grasses. They ranged in size at the shoulder from about 10 feet—slightly larger than an Asian Elephant—to 16 feet, taller than an African Elephant. The most obvious external characteristic of a mammoth is its long, curving tusks. Another type of Proboscidean, the mastodont, Mammut spp., of the extinct family Mammutidae, existed prior to and during the time of the mammoths. Unlike mammoths, the mastodonts had molar teeth with large cusps, resembling large human teeth. These teeth were adapted to crushing leaves and branches. Fossil bones and teeth of mastodonts have also been found in San Diego County. However, do not confuse the two—mammoths had long, curved tusks and flat-topped molars, whereas mastodonts had teeth with distinct, high cusps, and tusks much less curved than those of even modern elephants.

The discovery of the mammoth tusk at 16th and Market Streets was very exciting because never before had a fossil mammoth tusk been found in downtown San Diego. Prior to this discovery, mammoth fossil remains had been collected from 10 locations in San Diego County within the past 30 years, half of these within the last decade. Fossil tusks were only collected from four of these previous locations. Amazingly, another mammoth tusk was found just a few months later, increasing this total to five tusk discoveries at 11 collecting sites around the county. On December 10, 2007, Field Paleontologist Pat Sena of the Museum’s PaleoServices was monitoring the progress of the excavation of a bore hole at the SDG&E Silvergate Substation near the San Diego Bay harbor, just over a mile southeast of the corner of 16th and Market Streets.

A large boring auger had broken through the tusk 20 feet below street level near the bottom of the drilled hole. Sena collected the relatively small fragments and brought them to the Paleontology laboratory at the Museum where they were pieced back together.

Prior to the discoveries of fossil mammoth tusks at these two sites, other fossils had been discovered in downtown San Diego at numerous excavation projects. These fossils, found in great abundance, are shells occurring in sandy sedimentary layers, often a few feet thick. These fossil shell beds are positioned nearly horizontally, several feet beneath downtown San Diego. There are two main strata, or sedimentary sandstone layers, with minor intermediate beds containing less pronounced shell beds.